?? 3 dead chickens in the past 6 weeks....

all are 2 yr old leghorns bought at the feed store,, they become listless, and died w/in 24 hrs. no other flock members have any similar problems... i have several breeds... all of the feed store hens have slowed in production of eggs... they get commercial layer feed from a feed store, and nutritional snacks from me.... these have been random hens, now and then and nothing flock wide.... they just seem like they are giving up.... help